This document provides an introduction to genetics, summarizing key concepts and discoveries in the field's history. It discusses Gregor Mendel's foundational work with pea plants in the 1860s establishing the principles of heredity. It then summarizes the rediscovery of Mendel's work in 1900 and the subsequent discoveries that genes are located on chromosomes and that chromosomes are passed from parents to offspring according to Mendel's laws of inheritance. The document outlines concepts such as dominant and recessive traits, and how genetic inheritance works through the transmission of genes from parents to offspring.

2. A heartly thanks to all the teachers of science and
technology club to provide me this opportunity to give
presentation & thanks to the editors of Encarta & also
thanks to Nancy Hamilton Research team for providing
such good sources to students which are very helpful in
making projects and presentations.

3. CONTENTS
 INTRODCTION
 EMERGENCE OF GENETICS
 PHYSICAL BASIS OF HEREDITY
 THE TRANSMISSION OF GENES
 QUANTITATIVE INHERITANCE
 GENE LINKAGE AND GENE MAPPING
 GENE ACTION : DNA AND THE CODE OF LIFE
 CYTOPLASMIC INHERITANCE
 FURDER READING

4. INTRODCTION
 Genetics, scientific study of how physical, biochemical, and
behavioural traits are transmitted from parents to their offspring.
The word itself was coined in 1906 by the British biologist William
Bateson. Geneticists determine the mechanisms of inheritance
whereby the offspring of sexually reproducing organisms do not
exactly resemble their parents, and the differences and
similarities between parents and offspring recur from generation
to generation in repeated patterns. The investigation of these
patterns has led to some of the most exciting discoveries in
modern biology.
Gregor Mendel
Known as the father of modern genetics, Gregor
Mendel developed the principles of heredity while
studying seven pairs of inherited characteristics in pea
plants. Although the significance of his work was not
recognized during his lifetime, it has become the basis
for the present-day field of genetics.

5. EMERGENCE OF GENETICS
 The science of genetics began in 1900, when several
plant breeders independently discovered the work of the
Austrian monk Gregor Mendel, which, although published
in 1866, had been virtually ignored. Working with garden
peas, Mendel described the patterns of inheritance in
terms of seven pairs of contrasting traits that appeared in
different pea-plant varieties. He observed that the traits
were inherited as separate units, each of which was
inherited independently of the others (see Mendel's
Laws). He suggested that each parent has pairs of units
but contributes only one unit from each pair to its
offspring. The units that Mendel described were later
given the name genes.

6. PHYSICAL BASIS OF HEREDITY
 Soon after Mendel's work
was rediscovered,
scientists realized that the
patterns of inheritance he
had described paralleled
the action of
chromosomes in dividing
cells, and they proposed
that the Mendelian units of
inheritance, the genes, are
carried by the
chromosomes. This led to
intensive studies of cell
division.
Fruit Fly Chromosomes
The chromosomes of the fruit fly, Drosophila
melanogaster, lend themselves well to genetic
experiments. There are only 4 pairs—one of which,
marked here X and Y, determines the fly’s sex—
compared with the human complement of 23 pairs. In
addition, the fly’s chromosomes are very large. Thomas
Hunt Morgan and his associates based their theory of
heredity on studies using Drosophila. They found that
chromosomes were passed from parent to offspring in a
way that Gregor Mendel ascribed to inherited
characteristics. They proposed, correctly, that genes in
fact occupy specific physical locations on chromosomes.

7. Every cell comes from the division of a pre-existing cell. All the cells that make up a
human being, for example, are derived from the successive divisions of a single cell,
the zygote (see Fertilization), which is formed by the union of an egg and a sperm. The
great majority of the cells produced by the division of the zygote are, in the
composition of their hereditary material, identical to one another and to the zygote
itself (assuming that no mutations occur; see below). Each cell of a higher organism
is composed of a jellylike layer of material, the cytoplasm, which contains many small
structures. This cytoplasmic material surrounds a prominent body called the nucleus.
Every nucleus contains a number of minute, threadlike chromosomes. Some relatively
simple organisms, such as cyanobacteria and bacteria, have no distinct nucleus but
do have cytoplasm, which contains one or more chromosomes.
Chromosomes vary in size and shape and usually occur in pairs. The
members of each pair, called homologues, closely resemble each other. Most
cells in the human body contain 23 pairs of chromosomes, whereas most
cells of the fruit fly Drosophila contain four pairs, and the bacterium
Escherichia coli has a single chromosome in the form of a ring. Every
chromosome in a cell is now known to contain many genes, and each gene
is located at a particular site, or locus, on the chromosome.

8.  The process of cell division by which a new cell comes to have
an identical number of chromosomes as the parent cell is called
mitosis (see Reproduction). In mitotic division each
chromosome divides into two equal parts, and the two parts
travel to opposite ends of the cell. After the cell divides, each of
the two resulting cells has the same number of chromosomes
and genes as the original cell (see Cell: Division, Reproduction,
and Differentiation). Every cell formed in this process thus has
the same genetic material. Simple one-celled organisms and
some multicellular forms reproduce by mitosis; it is also the
process by which complex organisms achieve growth and
replace worn-out tissue.
Higher organisms that reproduce sexually are formed from the union of two
special sex cells known as gametes. Gametes are produced by meiosis, the
process by which germ cells divide. It differs from mitosis in one important
way: in meiosis a single chromosome from each pair of chromosomes is
transmitted from the original cell to each of the new cells. Thus, each gamete
contains half the number of chromosomes that are found in the other body
cells. When two gametes unite in fertilization, the resulting cell, called the
zygote, contains the full, double set of chromosomes. Half of these
chromosomes normally come from one parent and half from the other.

9. THE TRANSMISSION OF GENES
 The union of gametes brings together two sets of genes, one
set from each parent. Each gene—that is, each specific site
on a chromosome that affects a particular trait—is therefore
represented by two copies, one coming from the mother and
one from the father (for exceptions to this rule, see Sex and
Sex Linkage, below). Each copy is located at the same
position on each of the paired chromosomes of the zygote.
When the two copies are identical, the individual is said to be
homozygous for that particular gene. When they are
different—that is, when each parent has contributed a different
form, or allele, of the same gene—the individual is said to be
heterozygous for that gene. Both alleles are carried in the
genetic material of the individual, but if one is dominant, only
that one will be manifested. In later generations, however, as
was shown by Mendel, the recessive trait may show itself
again (in individuals homozygous for its allele).

10. .
Albinism
Albinism, the lack of normal pigmentation, occurs in all groups of
people. A rare condition, albinism occurs when a person inherits a
recessive allele, or group of genes, for pigmentation from each
parent. In this case, production of the enzyme tyrosinase is
defective. Tyrosinase is necessary to the formation of melanin,
the normal human skin pigment. Without melanin, the skin lacks
protection from the sun and is subject to premature ageing and
skin cancer. The eyes, too, colourless except for the red blood
vessels of the retina that show through, cannot tolerate light.
Albinos tend to squint even in normal indoor lighting and
frequently have vision problems. Tinted glasses or contact lenses
can help.
For example, the ability of a person to form pigment in the skin, hair, and eyes depends on the
presence of a particular allele (A), whereas the lack of this ability, known as albinism, is caused by
another allele (a) of the same gene. (For convenience, alleles are usually designated by a single
letter; the dominant allele is represented by a capital letter and the recessive allele by a small letter.)
The effects of A are dominant; of a, recessive. Therefore, heterozygous people (Aa), as well as people
homozygous (AA) for the pigment-producing allele, have normal pigmentation. People homozygous
for the allele that results in a lack of pigment (aa) are albinos. Each child of a couple who are both
heterozygous (Aa) has a probability of one in four of being homozygous AA, one in two of being
heterozygous Aa, and one in four of being homozygous aa. Only the individuals carrying aa will be
albino. Note that each child has a one-in-four chance of being affected with albinism; it is not
accurate to say that one-quarter of the children in a family will be affected. Both alleles will be carried
in the genetic material of heterozygous offspring, who will produce gametes bearing one or the other
allele. A distinction is made between the appearance, or outward characteristics, of an organism and
the genes and alleles it carries. The observable traits constitute the organism's phenotype, and the
genetic makeup is known as its genotype.

11. It is not always the case that one allele is dominant and the other recessive. The
four-o'clock plant, for example, may have flowers that are red, white, or pink.
Plants with red flowers have two copies of the allele R for red flower colour and
hence are homozygous RR. Plants with white flowers have two copies of the
allele r for white flower colour and are homozygous rr. Plants with one copy of
each allele, heterozygous Rr, are pink—a blend of the colours produced by the
two alleles.
The action of genes is seldom a simple matter of a single gene controlling a
single trait. Often one gene may control more than one trait, and one trait may
depend on many genes. For example, the action of at least two dominant genes
is required to produce purple pigment in the purple-flowered sweet pea. Sweet
peas that are homozygous for either or both of the recessive alleles involved in
the colour traits produce white flowers. Thus, the effects of a gene can depend
on which other genes are present.

12. QUANTITATIVE INHERITANCE
 Traits that are expressed as variations in quantity or extent, such as
weight, height, or degree of pigmentation, usually depend on many
genes as well as on environmental influences. Often the effects of
different genes appear to be additive—that is, each gene seems to
produce a small increment or decrement independent of the other genes.
The height of a plant, for example, might be determined by a series of
four genes: A, B, C, and D. Suppose that the plant has an average
height of 25 cm (10 in) when its genotype is aabbccdd, and that each
replacement by a pair of dominant alleles increases the average height
by approximately 10 cm (4 in). In that case a plant that is AABBccdd will
be 45 cm (18 in) tall, and one that is AABBCCDD will be 65 cm (26 in)
tall. In reality, the results are rarely as regular as this. Different genes
may make different contributions to the total measurement, and some
genes may interact so that the contribution of one depends on the
presence of another. The inheritance of quantitative characteristics that
depend on several genes is called polygenic inheritance. A combination
of genetic and environmental influences is known as multifactorial
inheritence.

13. GENE LINKAGE AND GENE
MAPPING
 Mendel's principle that genes controlling different
traits are inherited independently of one another
turns out to be true only when the genes occur on
different chromosomes. The American geneticist
Thomas Hunt Morgan and his co-workers, in an
extensive series of experiments using fruit flies
(which breed rapidly), showed that genes are
arranged on the chromosomes in a linear fashion;
and that when genes occur on the same
chromosome, they are inherited as a single unit for
as long as the chromosome itself remains intact.
Genes inherited in this way are said to be linked.

14. Perkin Elmer/Applied Biosytems
Division
Genetic Mapping
This gel scan showing the
arrangement of chromosomes within
a cell allows experts to take a closer
look at the genetic make-up of each
individual. With the completion of the
human genome project in 2005,
geneticists hope to compile a map
identifying and locating every gene
in the human body.
Morgan and his group also found, however, that such linkage is rarely complete. Combinations
of alleles characteristic of each parent can become reshuffled among some of their offspring.
During meiosis, a pair of homologous chromosomes may exchange material in a process called
recombination, or crossing-over. (The effect of crossing-over can be seen under a microscope
as an X-shaped joint between the two chromosomes.) Crossovers occur more or less at
random along the length of the chromosomes, so the frequency of recombination between two
genes depends on their distance from each other on the chromosome. If the genes are
relatively far apart, recombinant gametes will be common; if they are relatively close,
recombinant gametes will be rare. In the offspring produced by the gametes, the crossovers
show up as new combinations of visible traits. The more crossovers that occur, the greater the
percentage of offspring that show the new combinations. Consequently, by arranging suitable
breeding experiments, scientists can plot, or map, the relative positions of the genes along the
chromosome.

15. In recent years geneticists have used organisms such as bacteria, moulds, and viruses, which
rapidly produce extremely large numbers of offspring, to detect recombinations that occur only
rarely. Thus, they are able to make maps of genes that are quite close together. The method
introduced at Morgan's laboratory has now become so exact that differences occurring within a
single gene can be mapped. These maps have shown that not only do the genes occur in linear
fashion along the chromosome, but they themselves are linear structures. The detection of rare
recombinants can reveal the existence of structures even smaller than those observed through
the most powerful microscopes.
In recent years geneticists have used organisms such as bacteria, moulds, and viruses, which
rapidly produce extremely large numbers of offspring, to detect recombinations that occur only
rarely. Thus, they are able to make maps of genes that are quite close together. The method
introduced at Morgan's laboratory has now become so exact that differences occurring within a
single gene can be mapped. These maps have shown that not only do the genes occur in linear
fashion along the chromosome, but they themselves are linear structures. The detection of rare
recombinants can reveal the existence of structures even smaller than those observed through
the most powerful microscopes.
By March 2000, the entire genome (the complete set of genetic information) of the fruit fly had
been deciphered and mapped by another, faster method, whole-genome shotgun sequencing,
which splits the genome into tiny fragments and uses supercomputers to work out how these
fragments would reassemble and, therefore, the sequence of the fly’s genetic blueprint. This is
also one of the methods being used in the Human Genome Project (also see below).

16. GENE ACTION : DNA AND THE CODE OF
LIFE
 For more than 50 years after the science of genetics was
established and the patterns of inheritance through genes
were clarified, the largest questions remained unanswered:
how are the chromosomes and their genes copied from cell to
cell, and how do they direct the structure and behaviour of
living things? Two American geneticists, George Wells Beadle
and Edward Lawrie Tatum, provided one of the first important
clues in the early 1940s. Working with the fungi Neurospora
and Penicillium, they found that genes direct the formation of
enzymes through the units of which they are composed. Each
unit (a polypeptide) is produced by a specific gene. This work
launched studies into the chemical nature of the gene and
helped to establish the field of molecular genetics

17. That chromosomes were almost entirely composed of two kinds of chemical substances, protein
and nucleic acids, had long been known. Partly because of the close relationship established
between genes and enzymes, which are proteins, protein at first seemed the fundamental
substance that determined heredity. In 1944, however, the Canadian bacteriologist Oswald
Theodore Avery proved that deoxyribonucleic acid (DNA) performed this role. He extracted DNA
from one strain of bacteria and introduced it into another strain. The second strain not only
acquired characteristics of the first but passed them on to subsequent generations. By this time
DNA was known to be made up of substances called nucleotides. Each nucleotide consists of a
phosphate, a sugar known as deoxyribose, and any one of four nitrogen-containing bases. The four
nitrogen bases are adenine (A), thymine (T), guanine (G), and cytosine (C).
In 1953, putting together the accumulated chemical knowledge, geneticists James Dewey Watson
of the United States and Francis Harry Compton Crick of Great Britain worked out the structure of
DNA. This knowledge immediately provided the means of understanding how hereditary
information is copied. Watson and Crick found that the DNA molecule is composed of two long
strands in the form of a double helix, somewhat resembling a long, spiral ladder. The strands, or
sides of the ladder, are made up of alternating phosphate and sugar molecules. The nitrogen
bases, joining in pairs, act as the rungs. Each base is attached to a sugar molecule and is linked
by a hydrogen bond to a complementary base on the opposite strand. Adenine always binds to
thymine, and guanine always binds to cytosine. To make a new, identical copy of the DNA
molecule, the two strands need only unwind and separate at the bases (which are weakly bound);
with more nucleotides available in the cell, new complementary bases can link with each
separated strand, and two double helixes result. If the sequence of bases were AGATC on one
existing strand, the new strand would contain the complementary, or “mirror image”, sequence
TCTAG. Since the “backbone” of every chromosome is a single long, double-stranded molecule of
DNA, the production of two identical double helixes will result in the production of two identical
chromosomes.

18. The DNA backbone is actually a great deal longer than the chromosome but is tightly coiled up
within it. This packing is now known to be based on minute particles of protein known as
nucleosomes, just visible under the most powerful electron microscope. The DNA is wound
around each nucleosome in succession to form a beaded structure. The structure is then further
folded so that the beads associate in regular coils. Thus, the DNA has a “coiled-coil”
configuration, like the filament of an electric light bulb.
After the discoveries of Watson and Crick, the question that remained was how the
DNA directs the formation of proteins, compounds central to all the processes of life.
Proteins are not only the major components of most cell structures, they also control
virtually all the chemical reactions that occur in living matter. The ability of a protein to
act as part of a structure, or as an enzyme affecting the rate of a particular chemical
reaction, depends on its molecular shape. This shape, in turn, depends on its
composition. Every protein is made up of one or more components called
polypeptides, and each polypeptide is a chain of subunits called amino acids. Twenty
different amino acids are commonly found in polypeptides. The number, type, and
order of amino acids in a chain ultimately determine the structure and function of the
protein of which the chain is a part.

19. CYTOPLASMIC INHERITANCE
 Some constituents of the cell besides the nucleus contain DNA.
They include the cytoplasmic bodies known as mitochondria (the
energy producers of the cell) and the chloroplasts of plants,
where photosynthesis takes place. These bodies are self-
reproducing. The DNA is replicated in a manner similar to that in
the nucleus, and sometimes its code is transcribed and translated
into proteins. In 1981 the entire sequence of nucleotides in the
DNA of a mitochondrion was determined; apparently,
mitochondria use a code only slightly different from that used by
the nucleus.
 The traits determined by cytoplasmic DNA are more often
inherited through the mother than through the father (exclusively
through the mother in the case of Homo sapiens), because sperm
and pollen usually contain less cytoplasmic material than do
eggs. Some cases of apparent maternal inheritance are actually
due to the transmission of viruses from mother to offspring
through the egg cytoplasm.

20. FURDER READING
 Burns, George W. The Science of Genetics. 6th ed., 1989. Collier
Macmillan. Standard scientific textbook.
 Gribbin, John. In Search of the Double Helix: Quantum Physics and Life.
New York: McGraw-Hill, 1985. Examines molecular genetics, evolution,
physics.
 Griffiths, Anthony J. F., and McPherson, Joan. 100+ Principles of
Genetics. New York; Oxford: W. H. Freeman, 1989. Clearly organized
scientific text.
 King, Robert C., and Stansfield, William D. A Dictionary of Genetics. 3d
ed., 1985. Oxford University Press. Standard reference work.
 King, Robert C., and Stansfield, William D. Encyclopedic Dictionary of
Genetics. VCH, 1990. Comprehensive specialist reference work.
 Koestler, Arthur. The Case of the Midwife Toad. 1973. New York:
Random House, 1971. Implications of controversy surrounding
Kemmerer's principle of genetic regression
 Lloyd, J. R. Genes and Chromosomes. London: Macmillan, 1986.
Concise and clear, for student or general reader.
 Maclean, Norman. Macmillan Dictionary of Genetics & Cell Biology.
London: Macmillan, 1987. Standard reference dictionary.
 Singer, Sam. Human Genetics. Freeman, 1985. Fundamentals and
advances.